Alloys for Cast Metal Restorations Flashcards
What are the two alloys used for cast metal restorations?
- Crown and bridge alloys- Porcelain fused-to-metal alloys
What is a PFM crown?
- Porcelain surface- Metal alloy substructure - Porcelain fused metal crown
Why must porcelain be bonded to a metal alloy?
- Porcelain has good aesthetics - But microcracks tend to form at fitting surface due to large biting forces - Makes it prone to mechanical failure- Alloys withstand large stresses readily as have good mechanical properties
What is compressive strength?
- Stress to cause fracture
What is elastic modulus (rigidity)?
- Stress/strain ratio- Stress required to cause change in shape
What is brittleness/ductility?
- Dimensional change experienced before fracture
What is hardness?
- Resistance of surface to indentation or abrasion
What can be ascertained from a stress -strain curve and what can’t?
- Strength (compressive/tensile)- Brittleness/ductility- Elastic modulus (rigidity)- Hardness is not
What are some key features of a stress strain curve?
- Initial gradient = Elastic modulus (steeper gradient means more rigid)- Red dots at end of curve = fracture stress- If small gap between fracture stress and proportional limit = brittle- If large gap between FS and PL = ductile
What materials tend to be brittle and what tends to be ductile?
Brittle = CeramicsDuctile = Alloys
What are the properties of procelain?
- Hard so surface withstands abrasion/indentation well- Quite rigid so large stress required to cause strain- Strong so high compressive strength- Low tensile strength so tendency to form surface defects leads to fracture at low stress- Quite brittle so low fracture toughness- Not ductile- Maximum strain is approx 0.1% before fracturing
What are the properties of porcelain-fused alloys?
- Alloys much stronger- Much harder and more rigid- More ductile- Withstand greater degrees of permanent strain when subjected to large stresses like biting
What is the structure of porcelain-metal restorations?
- Metal oxide bonded to both the porcelain and the alloy
What is the purpose of the metal oxide?
- Helps eliminate defects/cracks on porcelain surface
What is the purpose of the alloy?
- Alloy supports and limits strain that porcelain experiences- More rigid so change shape very little and return to original dimensions- Helps it not reach level for brittle failure
To avoid developing defects or micro-cracks what must the porcelain-fused-ally undergo?
- Both porcelain and alloy should have similar thermal expansion coefficients - Due to needing to be fired in furnace then cooling - If didn’t have similar then defects would occur- So they expand at same rate when heated and contract at same rate when cooled
What are some different alloys that have been developed for bonding to porcelain?
- High gold alloy- Low gold alloy- Silver palladium (AgPd)- Nickel chromium (NiCr)- Cobalt chromium
Why must the alloy form a good bond to porcelain?
- AKA good wetting- Means goo surface contact- Porcelain forms bond with metallic oxides on surface- To ensure the restoration does not fall apart in patients mouth and swallow
Why must the thermal expansion coefficient of alloy be similar to porcelain?
- Porcelain is 14ppm per degree C- To avoid setting up stresses during fusing of porcelain to alloy- So defects and micro-cracks don’t form on cooling stage
What must a property of the alloy be to ascertain good aesthetics?
- Needs to avoid discolouration of porcelain as porcelain has good aesthetics- Ag in AgPd can produce green discolouration- Copper not used in high gold alloy for this reason
What mechanical properties are desirable of an alloy?
- High bond strength- Good hardness- High elastic modulus (rigid) to support porcelain and prevent fracture
What must the melting, recrystallisation temp of alloy be compared to porcelain?
- Must be higher than fusion temp of porcelain- Or creep may occur
What is creep?
- Gradual increase in strain (permanent) experienced under prolonged application of stress (
What are the constituents of High gold alloys?
- Gold (Au) = 80%- Platinum/Palladium (Pt/Pd) = 14% - Silver (Ag) = 1%- Small amount Indium, Tin- No Copper (green hue)
Why is Pt/Pd used in high gold alloys?
- Matches thermal expansion of porcelain- Increases its melting point which Helps minimise potential for creep
Why is Indium and Tin used in high gold alloys?
- Enable metal oxide layer to form- Enables bonding to porcelain
What are the disadvantages of high gold alloys?
- Melting range too low- Young’s modulus too low (not rigid)
What are the constituents for low gold alloy?
Au = 50%Pd = 30%Ag = 10%Indium, Tin = 10%
Why is low gold better than high gold?
- Increased melting temperature- Slightly better mechanical properties
What are the constituents of silver palladium alloys?
Pd = 60%Ag = 30%Indium, Tin = 10%
A negative and positive about silver palladium alloys?
- High melting point- Care needed in casting and is challenge for technicians
What are the constituents of Nickel-chromium alloys?
Ni = 70-80%Cr = 10-25% (creates oxide bond)
What are the advantages and disadvantages for Nickel-chromium alloy?
- High melting point- High young modulus (rigid)But - has high casting shrinkage so challenging to use- Low-ish bond strength to porcelain
What are the advantages and disadvantages to Cobalt-chromium alloys?
- High melting point (1300C-1400C)- High young modulus (220 GPa)- High tensile strength (850MPa)- High hardness (360-430 VHN)But- Low ish bond strength (220GPa)- Casting shrinkage 2.3%
Why are there biocompatibility concerns for nickel chromium?
- Allergy to nickel
What alloys have high casting shrinkage?
- AgPd- NiCr- Cocr
What is the most used alloy in GDH labs?
- CoCr
What are the 3 mechanisms for bond between oxide layer, alloy and procelain?
- Mechanical- Stressed skin- Chemical
What is the mechanical bond between oxide layer, alloy and porcelain?
- Due to surface irregularities on alloys metal oxide layer and porcelain- Allows them to interlock- Least important of the 3
What is the stressed skin effect?
- Depends on slight differences in thermal contraction coefficients- After furnace stage, alloy contracts slightly on cooling- Generates compressive forces on porcelain- Aids bonding
What is the chemical bond?
- Electron sharing in metal oxide coating alloy and the porcelain- Occurs during firing of porcelain where high temps reached- Described as electron sharing
What are the modes of failure in porcelain fusion?
- Oxide layer fracturing- Oxide layer delaminating from alloy- Porcelain detaching from oxide layer (most ideal situation)
